Hydrothermal vents are openings on the ocean floor where hot water flows out. This water is heated by the Earth's internal heat. These vents are often found near areas with volcanic activity, such as mid-ocean ridges, ocean basins, and hotspots where tectonic plates move apart. When hot water spreads out into the ocean from active vent sites, it forms hydrothermal plumes. Hydrothermal deposits are rocks and mineral deposits created by the action of hydrothermal vents.
Hydrothermal vents exist because the Earth is geologically active and has large amounts of water on its surface and inside its crust. Under the ocean, these vents can form features called black smokers or white smokers, which release many elements into the ocean. This helps shape the chemical balance of the world's oceans. Compared to most deep-sea areas, regions near hydrothermal vents have more life. These areas often support complex communities of organisms that rely on chemicals in the vent water. Chemosynthetic bacteria and archaea near vents form the base of the food chain, supporting life such as giant tube worms, clams, limpets, and shrimp. Scientists believe active hydrothermal vents may exist on Jupiter's moon Europa and Saturn's moon Enceladus. It is also thought that ancient hydrothermal vents once existed on Mars.
Some scientists suggest hydrothermal vents may have played a key role in the beginning of life on Earth. The conditions in these vents have been shown to help create molecules important for life. Certain types of vents, such as alkaline hydrothermal vents or those with supercritical CO₂, may be especially helpful in forming these molecules. However, the origin of life is a topic that scientists still debate, and there are many different ideas about how life began.
Physical properties
Hydrothermal vents in the deep ocean usually form along mid-ocean ridges, such as the East Pacific Rise and the Mid-Atlantic Ridge. These are places where two tectonic plates are moving apart, and new ocean floor is created.
The water that comes out of hydrothermal vents on the seafloor is mostly seawater that has moved into the vent system near a volcano through cracks and porous rock or volcanic layers. It also includes some water from magma rising underground. On land, most water in fumaroles and geysers comes from rainwater and groundwater that has seeped into the system from the surface. This water may also include some water from rocks, magma, or saltwater from deep within the Earth. The amounts of each type of water vary depending on the location.
The water around these vents is usually about 2°C (36°F), but the water that comes out of the vents can be much hotter, from 60°C (140°F) up to 464°C (867°F). Because of the high pressure at these depths, water can exist as a liquid or as a supercritical fluid at these temperatures. A supercritical fluid is a special state of water that has properties between a liquid and a gas. Pure water becomes supercritical at 375°C (707°F) and 218 atmospheres of pressure.
Adding salt to the water changes the conditions needed for it to become supercritical. Seawater, which has about 3.2% salt, becomes supercritical at 407°C (765°F) and 298.5 bars of pressure, which is about 2,960 meters (9,710 feet) below the ocean surface. If a hydrothermal fluid with this salt level vents at temperatures and pressures above these values, it is supercritical. The salt content of vent fluids can vary because of changes in the rock layers. Lower salt fluids have lower critical points than seawater but higher than pure water. For example, a fluid with 2.24% salt becomes supercritical at 400°C (752°F) and 280.5 bars. This means that water from the hottest parts of some vents can be supercritical.
Examples of supercritical venting have been found at several locations. At Sister Peak, a vent in the Comfortless Cove Hydrothermal Field, low-salt vapor-type fluids were observed. A short burst of water at 464°C (867°F) was recorded, but sustained supercritical venting was not found. A nearby site, Turtle Pits, released low-salt fluid at 407°C (765°F), which is above the critical point for that salt level. In the Cayman Trough, a site called Beebe, located about 5,000 meters (16,000 feet) below the ocean surface, has shown continuous supercritical venting at 401°C (754°F) with 2.3% salt.
Although supercritical conditions have been observed at multiple sites, scientists do not yet know the importance of supercritical venting for processes like water movement, mineral formation, chemical exchanges, or life in the deep ocean.
The first stage of a vent chimney begins with the formation of the mineral anhydrite. Over time, copper, iron, and zinc sulfides deposit in the gaps of the chimney, making it less porous. Some vent chimneys can grow as much as 30 centimeters (1 foot) each day. In April 2007, an exploration near Fiji found that these vents are a major source of dissolved iron, which plays a role in the iron cycle.
Black smokers and white smokers
Some hydrothermal vents create tall, chimney-like structures. These chimneys form when minerals dissolved in hot vent water mix with cold seawater. This mixing causes the minerals to come out of the water as particles, building up the chimneys. Some chimneys can grow as tall as 60 meters (200 feet). One example is "Godzilla," a chimney near Oregon in the Pacific Ocean that reached 40 meters (130 feet) before collapsing in 1996.
A black smoker is a type of hydrothermal vent found on the ocean floor, often in areas between 2,500 to 3,000 meters (8,200 to 9,800 feet) deep, though they can also be found in shallower or deeper regions. These vents look like dark, chimney-like structures that release a cloud of black material. Black smokers release particles rich in sulfur-containing minerals called sulfides. They form in groups when superheated water from Earth’s crust rises through the ocean floor, sometimes reaching temperatures above 400°C (752°F). This water carries dissolved minerals, especially sulfides, from the crust. When it meets cold seawater, the minerals form black, chimney-like structures. Heat from the water helps these structures grow thicker over time. Over time, the metal sulfides deposited can form large ore deposits. For example, some black smokers near the Azores in the Mid-Atlantic Ridge contain high amounts of dissolved iron, such as up to 24,000 μM in the Rainbow Vent Field.
Black smokers were first discovered in 1979 on the East Pacific Rise by scientists from the Scripps Institution of Oceanography during the RISE Project. They used a deep-sea vehicle called ALVIN from the Woods Hole Oceanographic Institution. Today, black smokers are found in the Atlantic and Pacific Oceans, usually at about 2,100 meters (6,900 feet) deep. The northernmost black smokers are a group called Loki’s Castle, found in 2008 near the Mid-Atlantic Ridge between Greenland and Norway at 73°N. These vents are interesting because they are in a more stable part of Earth’s crust, where tectonic activity is less common. The deepest known black smokers are in the Cayman Trough, 5,000 meters (3.1 miles) below the ocean surface.
White smoker vents release lighter-colored minerals, such as those containing barium, calcium, and silicon. These vents often have cooler plumes because they are farther from their heat source.
Black and white smokers may appear together in the same hydrothermal field, but they usually form near and far from the main heat source, respectively. White smokers are often found in later stages of hydrothermal activity when the heat source becomes less active. At this stage, fluids are mostly seawater, and the minerals deposited are rich in calcium, forming sulfate (like barite and anhydrite) and carbonate deposits.
Hydrothermal plumes
Hydrothermal plumes are fluid masses that form when hot fluids from hydrothermal vents are released into the surrounding seawater. These fluids often have different physical properties, such as temperature and density, and chemical properties, such as pH and ion levels, compared to seawater. These differences create chemical and physical changes that support various chemical reactions, such as oxidation-reduction and precipitation reactions.
Hydrothermal vent fluids are much hotter than seawater (ranging from about 40 to over 400 °C), while seawater near the ocean floor is about 4 °C. This temperature difference makes hydrothermal fluids less dense than seawater, causing them to rise due to buoyancy, forming a hydrothermal plume. This rising stage is called the "buoyant plume" phase. As the plume rises, mixing with seawater creates turbulence, which gradually dilutes the plume. Eventually, the plume becomes neutrally buoyant, meaning it no longer rises but spreads horizontally across the ocean, sometimes over thousands of kilometers. This stage is called the "nonbuoyant plume" phase.
Chemical reactions occur alongside the physical changes in hydrothermal plumes. Seawater is generally an oxidizing fluid, while hydrothermal vent fluids are often reducing. This difference causes reduced chemicals like hydrogen gas, hydrogen sulfide, methane, and metals such as iron (Fe) and manganese (Mn) to react when mixed with seawater. In fluids with high hydrogen sulfide levels, dissolved metals like Fe and Mn can form dark-colored sulfide minerals (e.g., "black smokers"). Over time, Fe and Mn in the plume may oxidize, forming insoluble (oxy)hydroxide minerals. For this reason, the area near hydrothermal vents where active metal oxidation occurs is called the "near field," while the region where complete metal oxidation has occurred is called the "far field."
Scientists use chemical tracers in hydrothermal plumes to locate deep-sea hydrothermal vents. Effective tracers should remain chemically unchanged after venting, so changes in their concentration are due only to dilution. Noble gas helium is a useful tracer because hydrothermal venting releases high levels of helium-3, a rare isotope from Earth's interior. This creates unusual helium isotope patterns in seawater that indicate hydrothermal activity. Another noble gas, radon, can also serve as a tracer. Radon isotopes are radioactive, and their concentrations, combined with helium isotope data, can help estimate the age of hydrothermal plumes. Radon-222 is particularly useful because it has the longest half-life among radon isotopes (about 3.82 days). Other substances, such as hydrogen gas, hydrogen sulfide, methane, and metals like Fe and Mn, may also indicate hydrothermal plumes but are less reliable as tracers because they are reactive and change over time.
Hydrothermal plumes play a key role in how hydrothermal systems affect marine biogeochemistry. Hydrothermal vents release many trace metals into the ocean, including iron (Fe), manganese (Mn), chromium (Cr), copper (Cu), zinc (Zn), cobalt (Co), nickel (Ni), molybdenum (Mo), cadmium (Cd), vanadium (V), and tungsten (W), many of which are important for biological processes. Once these metals enter the water column, their behavior is influenced by physical and chemical processes. According to thermodynamic principles, Fe and Mn should oxidize in seawater, forming insoluble metal (oxy)hydroxide precipitates. However, organic compounds and the formation of colloids or nanoparticles can keep these metals suspended in solution far from the vent site.
Iron and manganese often have the highest concentrations in acidic hydrothermal vent fluids and are biologically significant, especially iron, which is often a limiting nutrient in marine environments. The long-distance transport of Fe and Mn through organic complexation may be an important part of ocean metal cycling. Additionally, hydrothermal vents supply significant amounts of other biologically important trace metals, such as molybdenum, which may have played a role in early Earth ocean chemistry and the origin of life. However, Fe and Mn precipitates can also affect ocean biogeochemistry by removing trace metals from seawater. The charged surfaces of iron (oxy)hydroxide minerals can adsorb elements like phosphorus, vanadium, arsenic, and rare earth metals from seawater. Thus, while hydrothermal plumes may add metals like Fe and Mn to the ocean, they can also remove other metals and nutrients like phosphorus, acting as a net sink for these elements.
Biology of hydrothermal vents
Life has traditionally been thought to depend on energy from the sun, but deep-sea organisms near hydrothermal vents do not have access to sunlight. Instead, the biological communities around these vents rely on nutrients from chemical deposits and hydrothermal fluids. Earlier, scientists believed these organisms depended on marine snow, which comes from organic material falling from the ocean surface. This would mean they relied on plants and the sun. While some vent organisms do consume marine snow, this alone would not support a large number of life forms. However, hydrothermal vent zones have far more organisms than the surrounding seafloor—up to 10,000 to 100,000 times more.
Hydrothermal vents are a type of ecosystem where life uses chemicals instead of sunlight for energy. This process is called chemosynthesis. These ecosystems support many life forms because vent organisms depend on chemosynthetic bacteria for food. The water from hydrothermal vents contains dissolved minerals that support large populations of these bacteria. These bacteria use sulfur compounds, like hydrogen sulfide, which is toxic to most organisms, to create organic material through chemosynthesis.
Hydrothermal vents also provide iron to the ocean, which helps phytoplankton grow. The oldest known hydrothermal vent community is called Figueroa Sulfide, found in California during the Early Jurassic period. This ecosystem relies on the vent itself for energy, unlike most life on Earth, which depends on sunlight. However, some vent organisms still use oxygen made by photosynthetic life, while others do not need oxygen.
Chemosynthetic bacteria form thick mats that attract smaller animals, like amphipods and copepods, which eat the bacteria directly. Larger animals, such as snails, shrimp, crabs, tube worms, fish, and octopuses, form a food chain based on predator-prey relationships. The main groups of organisms found near hydrothermal vents include annelids, gastropods, and crustaceans, with large bivalves, tube worms, and eyeless shrimp being common.
Siboglinid tube worms, which can grow over 2 meters tall, are often found near vents. They lack mouths and digestive systems and absorb nutrients from bacteria inside their bodies. Each ounce of their tissue contains about 285 billion bacteria. These worms have red plumes that contain hemoglobin, which carries hydrogen sulfide to the bacteria. In return, the bacteria provide the worms with carbon-based nutrients. Two species of tube worms found near vents are Tevnia jerichonana and Riftia pachyptila. A community called "Eel City" is mostly made up of eels, located near a volcanic cone in American Samoa.
Over 100 gastropod species have been found near hydrothermal vents, and more than 300 new species have been discovered there. Many of these species are closely related to others found in different vent areas. Scientists believe that before the North American Plate moved over the mid-ocean ridge, all vent communities were part of a single region. This movement created barriers that led to the evolution of different species in separate areas. Similar traits found in vent communities from different locations support the theory of evolution.
Although life is sparse at great ocean depths, black smokers are the center of ecosystems. Sunlight does not reach these depths, so organisms like archaea and extremophiles use heat, methane, and sulfur compounds from black smokers to create energy through chemosynthesis. Larger animals, such as clams and tube worms, feed on these microbes. These microbes also deposit minerals into the black smoker, completing the life cycle.
A type of phototrophic bacterium was found near a black smoker off Mexico’s coast at a depth of 2,500 meters. This bacterium uses the faint light from the smoker instead of sunlight for photosynthesis. This is the first known organism to use non-solar light for photosynthesis.
New species are often found near black smokers. For example, the Pompeii worm (Alvinella pompejana), which can survive temperatures up to 80°C, was discovered in the 1980s. The scaly-foot gastropod (Chrysomallon squamiferum), found in 2001, uses iron sulfides to build its hard shell instead of calcium carbonate. The extreme pressure at these depths may help stabilize the iron sulfides for biological use. This armor likely protects the worm from predators.
In 2017, scientists found evidence of possible ancient life in hydrothermal vent deposits in Quebec, Canada. These fossils may be over 4.28 billion years old, dating back to when the Earth was very young.
Hydrothermal vent ecosystems have high levels of life, but this depends on the relationships between organisms. Unlike shallow-water or land-based hydrothermal systems, deep-sea vents rely on symbiosis between animals and chemosynthetic bacteria. Since sunlight does not reach these depths, life uses chemicals like hydrogen sulfide instead of sunlight for energy. These bacteria convert inorganic molecules into organic ones, which the host animals use for food. Sulfide is toxic to most life, so scientists were surprised to find so many organisms thriving near vents. They discovered that these animals have bacteria living inside their bodies, which help them survive the toxic environment. Scientists are now studying how these bacteria protect their hosts from sulfide’s harmful effects.
Discovery and exploration
In 1949, a study of deep ocean water in the central Red Sea found unusually hot saltwater. In the 1960s, scientists confirmed the presence of hot, 60 °C (140 °F) saltwater and muddy deposits rich in metals. These hot waters were coming from an active crack in the seafloor. The extremely salty water made it hard for living things to survive. Scientists are now studying these brines and muds to see if they can be used to mine valuable metals like gold and copper.
In June 1976, scientists from the Scripps Institution of Oceanography discovered the first evidence of underwater hydrothermal vents along the Galápagos Rift, a branch of the East Pacific Rise, during the Pleiades II expedition. They used a deep-sea imaging system called Deep-Tow. In 1977, scientists from Scripps published the first scientific reports about these vents. Researcher Peter Lonsdale shared photos taken by deep-sea cameras, and student Kathleen Crane shared maps and temperature data. Scientists placed devices called transponders at the site, nicknamed "Clam-bake," to help return for further study the next year using the submersible DSV Alvin.
In 1977, scientists from the National Science Foundation directly observed ecosystems near the Galápagos Rift vents. Jack Corliss of Oregon State University led the study. Corliss and Tjeerd van Andel from Stanford University explored the vents and their ecosystems in the DSV Alvin. Other scientists on the expedition included Richard Von Herzen and Robert Ballard from Woods Hole Oceanographic Institution (WHOI), Jack Dymond and Louis Gordon from Oregon State University, John Edmond and Tanya Atwater from MIT, Dave Williams from the U.S. Geological Survey, and Kathleen Crane from Scripps. They published their findings in the journal Science. In 1979, biologists led by J. Frederick Grassle from WHOI returned to the same area to study the life forms found earlier.
High-temperature hydrothermal vents, called "black smokers," were discovered in spring 1979 by scientists from Scripps using the submersible Alvin. The RISE expedition explored the East Pacific Rise near 21° N to test mapping techniques and find new vent fields beyond the Galápagos Rift. The expedition was led by Fred Spiess and Ken Macdonald, with participants from the U.S., Mexico, and France. The dive area was chosen based on discoveries of sulfide mineral mounds by the French CYAMEX expedition in 1978. Before diving, Robert Ballard used a deep-towed instrument to find temperature differences in the water. The first dive targeted one of these areas. On April 15, 1979, during a dive to 2,600 meters, Roger Larson and Bruce Luyendyk found a vent field with life similar to the Galápagos vents. On April 21, William Normark and Thierry Juteau discovered the black smokers, which released black mineral particles from chimneys. Macdonald and Jim Aiken attached a temperature probe to Alvin, measuring the highest temperatures ever recorded at that time (380±30 °C). Analysis of black smoker material showed that iron sulfide is the main mineral in the "smoke" and chimney walls.
In 2005, Neptune Resources NL, a mineral exploration company, received permission to study 35,000 km of the Kermadec Arc in New Zealand’s Exclusive Economic Zone for seafloor sulfide deposits, which could be a new source of lead, zinc, and copper. In 2007, scientists announced the discovery of a vent field off Costa Rica, named the Medusa hydrothermal vent field. The Ashadze hydrothermal field, located at 13°N on the Mid-Atlantic Ridge at -4200 m, was the deepest known high-temperature vent field until 2010. That year, scientists from NASA and WHOI discovered a hydrothermal plume at the Beebe site, located on the Mid-Cayman Rise at -5000 m. In 2013, the deepest known hydrothermal vents were found in the Caribbean Sea at nearly 5,000 meters (16,000 feet).
Oceanographers are studying volcanoes and hydrothermal vents on the Juan de Fuca mid-ocean ridge, where tectonic plates are moving apart. Scientists are also exploring hydrothermal vents and other geothermal features in Bahía de Concepción, Baja California Sur, Mexico.
Distribution
Hydrothermal vents are found along the edges where Earth's plates meet, though some are also located within the plates, such as at hotspot volcanoes. As of 2009, about 500 active submarine hydrothermal vent fields were known. Approximately half of these were seen directly on the seafloor, while the other half were suspected based on signs in the water or deposits on the seafloor.
In 2012, Rogers and others identified at least 11 different regions of hydrothermal vent systems:
- Mid-Atlantic Ridge region,
- East Scotia Ridge region,
- northern East Pacific Rise region,
- central East Pacific Rise region,
- southern East Pacific Rise region,
- south of the Easter Microplate,
- Indian Ocean region,
- four regions in the western Pacific, and many more.
Exploitation
Hydrothermal vents sometimes create mineral deposits that can be used as resources. These deposits, called seafloor massive sulfide deposits, form when minerals are released from the vents. One example is the Mount Isa orebody in Queensland, Australia. Many hydrothermal vents contain valuable metals such as cobalt, gold, copper, and rare earth metals, which are used in electronics. Scientists believe that hydrothermal venting on the Archean seafloor formed Algoma-type banded iron formations, which are important sources of iron ore.
In the mid-2000s, companies exploring for minerals focused on extracting resources from hydrothermal fields on the ocean floor. This could potentially lower costs. In Japan, where most minerals are imported, there is strong interest in mining seafloor resources. In 2017, Japan Oil, Gas and Metals National Corporation (JOGMEC) conducted the world’s first large-scale mining of hydrothermal vent deposits. This operation took place at the Izena hole/cauldron vent field in the Okinawa Trough, a region with 15 confirmed vent fields.
Two companies are now working to begin mining seafloor massive sulfides (SMS). Nautilus Minerals is close to starting extraction at its Solwarra deposit in the Bismarck Archipelago. Neptune Minerals is earlier in the process at its Rumble II West deposit near the Kermadec Islands. Both companies plan to use modified technology. In 2006, Nautilus Minerals successfully brought over 10 metric tons of SMS to the surface using drum cutters attached to an underwater robot. In 2007, Neptune Minerals used a modified suction pump on an underwater robot to collect SMS samples.
Mining the seafloor may harm the environment. Dust from mining machines could affect filter-feeding organisms. Mining might also cause vents to collapse, release methane, or trigger underwater landslides. Tools used for mining, such as underwater robots and surface ships, can create noise and human-made light. Many deep-sea organisms live in very quiet, dark environments and have sensitive hearing. Sudden noise from mining could damage their hearing or disrupt communication. Some deep-sea animals use low-frequency sounds to communicate, so increased noise might interfere with their behavior. Human-made light from mining tools and ships could harm organisms adapted to darkness. Studies suggest that bright lights used to study vent systems might damage the eyes of deep-sea shrimp, raising concerns for other vent species. Light from surface ships might also disorient seabirds, causing them to collide with objects or become exhausted.
Three mining waste processes—side cast sediment release, dewatering, and sediment shift—could create sediment plumes, which are clouds of particles in the water. Side cast sediment release involves moving material from the seafloor using underwater robots, which might leave sediment near the mining site. Dewatering involves releasing water from ships, which may contain heavy metals like copper and cobalt. This could change ocean chemistry or harm marine life. Sediment shift occurs when mining activities disturb the seafloor, spreading sediment to other areas. This might smother organisms, disrupt feeding, or reduce gas exchange. Increased sediment on the seafloor could also raise sedimentation rates, affecting ecosystems.
Conservation
For the past 20 years, scientists and oceanographers have had intense debates about how to protect hydrothermal vents. Some people say that scientists might be the main cause of harm to these rare habitats. Efforts have been made to create rules for how scientists should behave when studying these areas. While there is a shared set of guidelines, there is no official international agreement that is legally required.
Protecting hydrothermal vent ecosystems after mining would depend on chemosynthetic bacteria returning to the area. These bacteria rely on hydrothermal vent fluid, which provides energy for the ecosystem. It is hard to know how mining affects this fluid because no large studies have been done. However, scientists have studied how vent ecosystems recover after volcanic events. These studies show that bacteria can return to an area in 3–5 years, and larger animals may take about 10 years to return. Researchers also found that the types of species in the ecosystem changed after destruction, with new species moving in. More research is needed to understand how long-term mining might affect species returning to these areas.
Geochronological dating
Scientists use specific methods to determine the ages of hydrothermal vents. These methods involve dating sulfide minerals, such as pyrite, and sulfate minerals, such as baryte. Common techniques include radiometric dating and electron spin resonance dating. Each method has its own limitations, assumptions, and challenges. Some general challenges include the need for highly pure minerals to be tested, the limited age ranges each method can measure, and the risk of heating minerals above closure temperatures, which can erase older age records. Additionally, if minerals form in multiple stages, the ages recorded may mix together. In such cases, electron spin resonance dating typically provides the average age of the entire mineral sample, while radiometric dating often reflects the ages of younger mineral layers due to the decay of parent elements. These factors explain why different dating methods may produce varying ages for the same sample and why samples from the same hydrothermal chimney can have different ages.
History and formation of hydrothermal vents
Some scientists, like Rogers et al. (2012), have found hydrothermal vents. However, the exact locations of these vents in deep sea areas are not well known. The ocean floor is mostly unexplored, with less than 1% of it studied in detail. Most of the hydrothermal vents scientists know about are found along mid-ocean ridges. Understanding where these systems are located helps scientists study how they form, as many theories about their creation involve seismic activity, especially near volcanic areas.
During the Paleocene and Eocene periods, seismic activity caused gases, liquids, and sediments from Earth’s core to erupt. This event formed large craters above layers of igneous rock called sills. Sills are rock layers created when molten rock pushes between existing rock layers. These craters on the seafloor contain groups of hydrothermal vents. Features of these vents include sediment layers that slope inward, as well as sandstone structures like dykes, pipes, and broken rock fragments. These features are called subvolcanic intrusions and lead to hydrothermal activity. A study used 2D seismic reflection data to describe these systems, which are located in craters with a funnel-like shape. These structures are often called chimneys, which form on top of the vents.
The interaction between the oceanic crust and seawater creates these systems. This process changes the local chemistry and forms deposits rich in different metals. These metal deposits and chemical changes create conditions that support life, such as thermophiles and other organisms.